DE69329599T2 - THERMOPLASTIC CORROSION PROTECTIVE LAYER FOR METALS - Google Patents

Description

BACKGROUND OF THE INVENTION Field of the invention

The invention relates to thermoplastic anti-corrosion coatings, in particular, base coats for metals. More particularly, it relates to copolymers of ethylene with a comonomer containing a glycidyl group, optionally with a filler such as zinc, which can be applied as a thermoplastic coating to prevent the corrosion of metals. The coating can be used alone as a sole coating, but is preferably used as a base coat under an outer coating providing mechanical protection for the base coat.

Discussion of technical background and related technology

Metal vessels, pipes and other forms used to store and transport a range of materials are subject to corrosion or erosion by the materials stored or transported. Metal objects are also subject to corrosion or erosion by the environment with which they come into contact. For example, soil, salt water or atmospheric and climatic conditions can stress the metal.

To protect them from such corrosion and erosion, metals are generally coated with plastic materials. In addition to providing protection from corrosion or erosion, certain plastic materials provide desirable properties inherent in the plastic used. For example, a very smooth surface can lower the coefficient of friction in a pipe, thus reducing the energy required to pump a fluid through the pipe.

To provide effective corrosion protection, a coating should have good adhesion to the metal and should be relatively impermeable to substances that could themselves cause corrosion of the metal or to substances that could cause loss of adhesion of the coating to the metal. Poor initial adhesion or subsequent loss of adhesion allows the metal itself to be directly exposed to corrosive environments. Therefore, both impermeability and long-term adhesion are important properties for a good corrosion protection coating.

However, coating materials vary in their merits. Thermoplastic polyolefin coatings, such as polyethylene or polypropylene, are water and chemical resistant, but they do not adhere well to metals. In contrast, ethylene copolymer ionomers, which are ion-neutralized ethylene-(meth)acrylic acid copolymers, provide a high degree of adhesion to metals, are tough and provide good abrasion resistance, but are sensitive and permeable to water.

Adhesion and the durability of this adhesion to metals are complicated phenomena. Loss of adhesion can have mechanical or chemical causes. Different thermal expansion of the metal and the coating can lead to mechanical failure of the bond between them, while many substances can attack the bond between metal and coating.

Since all the properties of a good coating (relative impermeability to potential rust formers plus good and durable adhesion under a wide range of conditions) are not always possible in a single coating, it is common to use primer coats between the metal and an outer plastic coating to provide durable adhesion between the metal and an outer coating, but retain the benefits of the outer coating.

Thermosetting epoxy primers or sole coatings are well known and are among the preferred materials for primers or sole coatings. Thermosets have the advantage of having a relatively low coefficient of expansion and less differential coefficient of expansion compared to metals. However, they are quite brittle and are therefore used in fairly thin layers. In addition, they must be cured. Thermosetting epoxies are excellent adhesives, but they do not necessarily provide ideal coatings for many purposes.

Particulate fillers, particularly those with a laminar shape, are commonly used in coatings to increase corrosion resistance. They help reduce differences in expansion coefficients and can reduce permeability by increasing the distance a fluid needs to travel to penetrate the coating. Particulate zinc as a filler is particularly beneficial because it has another anti-corrosive effect related to its reducing potential. Its use in coatings and paints is well known. The use of zinc per se as a protective coating has of course been known for a long time, especially in steel, due to its reducing or galvanizing potential.

US-3,201,497 (Heino) describes a thermosetting epoxy coating adhesive that can be used as a protective coating as well as for bonding metals together. Metals include steel, aluminum, copper and chromium. The epoxy resin may contain a comonomer such as glycidyl methacrylate which acts as a "reactive plasticizer" during the curing of the resin. A variety of fillers, including zinc dust, may be included in the adhesive composition.

US Patent 3,941,609 (Stern) describes the use of zinc powder with a controlled particle size distribution in paints to provide corrosion-inhibiting paints.

Thermoplastic coatings have certain advantages over thermosetting resins. They can be less brittle and have all the benefits of simple application techniques and do not require a separate curing step. On the other hand, thermoplastics have a high coefficient of elongation compared to many thermosets and therefore have limited applications. A thermoplastic coating is taught in UK Patent Specification No. 88/20807, published March 1990, which is a priority document for US Patent No. 5,091,260. The thermoplastic material, suitable as a sole coating or as a base coat, is based on blends of polyethylene or polypropylene grafted with maleic acid or anhydride containing zinc powder. These thermoplastic coatings can be applied using normal thermoplastic methods. These polyolefins grafted with maleic anhydride require a separate, costly grafting step.

There remains a need for a thermoplastic polymer composition that is well suited as a metal coating and/or metal base coat, that has adhesion properties comparable to thermosetting epoxy resins, and that is easy to prepare and apply to metal as a corrosion inhibitor.

SUMMARY OF THE INVENTION

It has now been found that it is possible to obtain adhesive and protective properties without the brittleness and curing needs of a thermosetting epoxy in a composition having a base of a thermoplastic polymer having a specific type of epoxy functionality. The thermoplastic coating of the invention contains an epoxy functionality which does not require curing and does not cause the polymer to heat set, but which provides a high degree of corrosion protection, particularly for steel.

In the invention there is provided a metal coating composition comprising a mixture of 18-100% by weight of an ethylene copolymer comprising 40-95% by weight of ethylene, 0-50% by weight of a comonomer selected from vinyl acetate, a C3 to C11 alkyl acrylate, alkyl methacrylate or alkyl vinyl ether, and 2-20% by weight of glycidyl acrylate, glycidyl methacrylate or glycidyl vinyl ether. Optionally, the composition may contain up to 82% by weight of a filler, particularly particulate zinc, based on the weight of copolymer and filler.

Further according to the invention there is provided a coated metal consisting of a metal layer, where the metal may be iron, steel or aluminum, a base coat of the metal coating composition described above, and an outer coating, over the base coat, of polyethylene or polypropylene or an ethylene-acrylic acid or methacrylic acid ionomer.

DETAILED DESCRIPTION OF THE INVENTION

The thermoplastic polymer of the invention is an ethylene copolymer with a glycidyl-containing comonomer. This material can be used as a sole coating, i.e. as the only coating, especially with a filler, but since it adheres well to both metal and other ethylene polymers or copolymers, it can serve as a base coat on metal. An outer coating of ethylene polymer or copolymer can be used over the primer. Preferably, it is used as a primer. Applications in which the thermoplastic polymer of the invention can be used without an outer coating are those in which mechanical damage to the relatively soft, pliable thermoplastic material is unlikely.

The glycidyl-containing ethylene copolymers are copolymers comprising 40-95 wt.% ethylene and 2-20 wt.% glycidyl-containing comonomer. With less than about 2 wt.% glycidyl-containing comonomer, the epoxy functionality is insufficient to provide a moisture-resistant, corrosion-resistant bond between the metal and the copolymer. With more than 20 wt.%, the copolymer becomes expensive and can result in fouling during copolymerization. The glycidyl-containing comonomer can be a glycidyl acrylate, a glycidyl methacrylate or a glycidyl vinyl ether, preferably glycidyl methacrylate.

Preferably, a second comonomer is also present. This second comonomer serves to reduce crystallinity compared to a polyethylene homopolymer without increasing the amount of the relatively expensive glycidyl comonomer. It may also serve to increase the low temperature toughness of the copolymer if the glass transition of its homopolymer is low. These second comonomers include vinyl acetate and alkyl acrylates, alkyl methacrylates and alkyl vinyl ethers. They may be present up to 50% by weight of all monomers in the copolymer. Above this amount, they are difficult to incorporate into the copolymerization. An intermediate amount of 5-30% by weight is preferred. The preferred second comonomer is n-butyl acrylate because this monomer forms a homopolymer with a low glass transition temperature.

These ethylene copolymers can be prepared by known methods for high pressure, free radical, ethylene copolymerization using a continuously stirred reactor operating under equilibrium conditions. Such a process is described in U.S. Pat. No. 3,780,140, which is hereby incorporated by reference.

Fillers may be present in the ethylene copolymer coatings of the invention. The shape, size and size distribution all affect the effectiveness of a filler, although at high levels the specific properties of the filler become less important. Zinc flakes and powders appear to be very suitable A small particle size facilitates the production of uniform coatings. For example, the particles preferably have a maximum diameter of less than about 400 micrometers, and most preferably less than 45 micrometers. The copolymer composition can be mixed with the filler by known mixing techniques using extruders or other suitable mixers such as continuous Banbury or Farrel mixers or roll mills.

The amount of filler, if present, can vary widely. At more than about 82% particulate filler, based on the weight of the copolymer plus filler, properties of the filled material such as flexibility, ductility, elongation and tensile strength drop rapidly. A small amount of filler (in some cases 2% and more and in others up to 5% and more) may provide sufficient benefits for some coating environments or applications, while in other cases large amounts (up to about 82%) of particulate filler, for example a reducing filler such as zinc, are preferable. One skilled in the art will be able to determine if and how much filler is most appropriate.

When coating metals with plastic coatings, it is common practice to first sandblast the metal and/or clean the metal surface with solvents to assist in the removal of grease or oxide layers. In addition, washing with various silanes, such as γ-aminopropyltriethoxysilane, can help reduce the damaging effects of moisture on the metal coating interface. Pretreatment of the metal is preferred.

The thermoplastic coatings of the invention can be applied to the metal surface by pressure lamination, vacuum lamination, extrusion coating, flame spraying or any other method suitable for thermoplastic coating. Thicker coatings, which generally provide better protection of the coated metal, can be applied without the problems resulting from the brittleness of the thermosetting epoxy resin. It However, it has been found that coatings as thin as 10 mils (0.25 millimeters) are perfectly adequate.

EXAMPLES

All percentages in the examples are by weight.

Preparing and testing samples

"Coated panels" for testing were prepared in the laboratory as follows: "Treated carbon steel panels" were prepared by first gritting and then washing with 1,1,1-trichloroethane and then treating with a solution of γ-aminopropyltriethoxysilane in 50/50 ethanol/water (0.2 to 0.4% solution).

When filler was used, it was mixed with polymer using a roller mill at a temperature of about 180ºC until a homogeneous mixture was obtained (typically about 10 minutes).

The polymer mixed with filler, if present, was formed into sheets by compression molding using a hydraulic press at a temperature of about 190°C and a pressure of about 2.76 x 10⁸ Pa to give a film of about 10 mils (0.25 mm).

The web was then laminated to the treated carbon steel plate using either vacuum forming techniques at 190°C and a vacuum of 8.94 x 10⁴ Pa, or pressing the webs to the treated carbon steel plate at 160°C with minimal pressure for about 3 minutes, followed by a pressure of 6.9 x 10⁴ Pa for 1 minute. If an outer coating was added, a 10 mil thick coating was laminated over it using similar techniques with minor adjustments. the temperature and pressure as required to obtain a suitable outer coating.

Testing

Due to the wide variety of possible applications and environments envisaged for the coated metals for which the coatings of the invention are used, no single test is adequate to predict corrosion protection capability. Several different tests were used to simulate a range of possible conditions. All coated panels were exposed to a sodium chloride solution or spray of sodium chloride at elevated temperatures for varying periods of time and tested in different ways. The tests used were as follows:

Sodium chloride immersion test:

The coated panel was immersed in either a 5% or 10% aqueous sodium chloride solution at 70ºC. The results appeared to be essentially the same regardless of which concentration was used. The adhesion versus time was then measured using the 180º peel test (ASTM D-903). Materials suitable for real-world application conditions are believed to last at least 200 hours under the test conditions and preferably longer with a measurable peel strength. Absolute correlations between the test conditions and the application conditions are, of course, very difficult to verify.

Cathode bonding solution test

Since metals, especially steel, are subject to oxide formation and the subsequent formation of galvanic cells with release of hydrogen, a useful test is common in testing laboratories that uses a low voltage applied to coated panels. In this test, the surface of the coated panel was first cut to remove a circle (a "pore") 0.635 centimeters (cm) in diameter, exposing the metal. The coated panel was then placed in a 3% sodium chloride solution at 60ºC and a voltage of -1.5 volts was applied to the coated panel. The coated panels were examined after different periods of time and the loss of adhesion was visually assessed. Some tests lasted 48 hours while others lasted 72 hours.

Salt spray test:

A vertical cut was made in the polymer coating of a coated panel with a knife or scriber until the metal was exposed. The structure was then placed in a vertical position in a salt spray chamber, which produces a mist of 5% sodium chloride solution, and maintained at 37.8ºC (100ºF). The coated panels were examined at weekly intervals for loss of adhesion. The coating was bent back at the cut to the point where adhesion still existed. The distance of metal surface then exposed was measured; coated panels showing delamination greater than 0.318 cm are considered to be defective.

The following examples illustrate the excellent corrosion resistance of coatings and primers of the invention.

COMPARISON EXAMPLES 1 AND 2

In Comparative Example 1, a 10 mil thick layer of a mixture of 5 wt.% zinc powder (Zinc Corporation of America, Grade No. 44, sized to retain 3% or less on a 325 mesh screen) and 95 wt.% of an ionomer based on an 85/15 ethylene/methacrylic acid copolymer, 57% neutralized with zinc, and having an MI of 0.8 g/10 min (as measured by ASTM D-1238 using 2160 g at 190ºC) was laminated to a treated carbon steel plate. In Comparative Example 2, 10 wt.% zinc was used in the same polymer. The ionomer/zinc blend coating was the only coating.

The coated panels were subjected to the sodium chloride immersion test above. Peel strength was 1.94 Newtons per centimeter (N/cm) after 96 hours and 0 N/cm after 240 hours for Comparative Example 1 and 2.47 N/cm after 96 hours and 0 N/cm after 240 hours for Comparative Example 2. Although a larger amount of zinc provided a slight improvement, both the level and the time that a measurable peel strength was maintained were very low. Although the ionomers generally adhered well to metal initially, they did not provide a coating that gave durable protection, even when zinc was present.

EXAMPLES 1 AND 2

A 10 mil thick base coat consisting of 5 wt% zinc in Example 1 and 10 wt% zinc in Example 2 (the same grade as in Comparative Examples 1 and 2) in a 63.7/28/8.3 ethylene/n-butyl acrylate/glycidyl methacrylate copolymer having a melt index of 12-15 g/10 min (as measured by ASTM D-1238 using 2160 g at 190°C) was laminated to a treated carbon steel panel. A 10 mil thick outer coat consisting of the ethylene/methacrylic acid based ionomer used in Comparative Examples 1 and 2 but not containing any zinc particles was then laminated on top of the base coat. The coated panels were subjected to the sodium chloride immersion test as in the comparative examples. In Example 1, the peel strength after 432 hours was still 30.04 N/cm, while in Example 2, the peel strength after 432 hours was 48.6 N/cm. Although a larger amount of zinc in the base coat may result in a slight improvement, the most dramatic difference is seen when a glycidyl-containing ethylene copolymer is used as the primer.

COMPARISON EXAMPLE 3

A coated panel was prepared using the Treated Carbon Steel Panel and a 10 mil thick coating of the ionomer of the above examples containing 70 wt% zinc dust (the same grade as above). The coated panel was tested using the Cathode Bond Dissolution Test. Complete bond dissolution occurred after 48 hours, even when a large amount of zinc dust was present.

EXAMPLE 3

A coated panel was prepared using a 10 mil thick coating of ethylene/n-butyl acrylate/glycidyl methacrylate with a weight ratio of 63.7/28/8.3 containing 65 wt% zinc flake with an average particle size of 840 microns and another 10 mil thick coating of linear low density polyethylene grafted with 0.8 wt% maleic anhydride. It was tested using the cathode bond dissolution test. After 72 hours, bond dissolution occurred only 3.1 mm from the edge of the pore. This test is a stress test and a large amount of zinc was used. However, it is clear that a high degree of protection from bond dissolution is provided when a glycidyl-containing ethylene copolymer is applied directly to the metal surface.

EXAMPLES 4 and 5

Coated panels were prepared as in Example 1 using a base coat of the same ethylene/n-butyl acrylate/glycidyl methacrylate terpolymer containing 65 and 80 wt% zinc flake (same as for Example 3) for Examples 4 and 5, respectively, and a top coat of the same ionomer as in Example 1. The coated panels were subjected to the salt spray test described above The coated panels showed no delamination at the scribe mark even after 5000 hours. This is considered an excellent level of durability. Comparable coatings based on ionomer filled with a similar amount of zinc fail within 24 hours.

Claims (5)

1. A method for protecting metal against corrosion, comprising applying to the metal a mixture comprising: (i) 18-100 wt.% unmodified ethylene copolymer comprising: (a) 40-95 wt% ethylene, (b) 0-50% by weight of a comonomer selected from the group consisting of vinyl acetate, alkyl acrylates, alkyl methacrylates and alkyl vinyl ethers, wherein the alkyl moiety has 3-11 carbon atoms, (c) 2-20 wt.% glycidyl acrylate, glycidyl methacrylate or glycidyl vinyl ether, and (d) 0-82 wt.% of a particulate filler. 2. The process of claim 1, wherein (b) is n-butyl methacrylate, and (c) is glycidyl methacrylate. 3. A process according to claim 1 or 2, wherein (i) is present in an amount of 18 to 98 wt% and (ii) is present in an amount of 2-82 wt%. 4. The method of claim 3, wherein the particulate filler is zinc. 5. The method of claim 4, wherein (a) is present in an amount of 45-90 wt.%, (b) in an amount of 5-30 wt.%, and (c) in an amount of 5-10 wt.%.